What version of LW are you using? You'll only be able to preview & launch files using VLC
within LW if you're using the 4.13.1 beta
For all other versions, you'll have to open the file outside of LW.
If you'd like to try the beta, there's a link at the bottom of the LW basic downloads page
http://www.limewire.com/english/content/download.shtml
Or, if you're a Pro user, it's available from your personal download page.